Positive And Negative Tb Skin Test – What’s The Difference

Such a large number of cases of Tuberculosis in India is clearly due to over-crowding seen commonly among slum dwellers. Although anybody can be infected with Tb. Poor living conditions and poor sanitation are some other predisposing factors for developing Tuberculosis.

Infection of lungs with Mycobacterium Tuberculosis is called Pulmonary Tuberculosis. When this bacillus infects some other parts of the body, it is called Extra Pulmonary Tuberculosis. Nearly one-third of the population worldwide is infected with tuberculosis, out of which 5-10% develop a clinical disease later in life.

When an infected person cough or sneezes, the bacteria gets expelled and can be transmitted to other people who inhale these droplets. Surprisingly, many people who are infected with Mycobacterium tuberculosis remain asymptomatic for several years. The bacteria tend to multiply rapidly in immuno-compromised individuals.

Plenty of investigations like a chest x-ray, Sputum Examination for Acid-Fast Bacillus (AFB) and Tuberculin test or a Tb skin test have been discovered for diagnosis of Tuberculosis.

Background and Statistics

At the time when Dr Robert Koch discovered the cause of Tuberculosis to be the bacillus Mycobacterium Tuberculosis. Tb had affected a vast majority of the population in America and in Europe and was responsible for a large number of deaths.

In India, even in the present time, eradication of Tuberculosis remains a major challenge for the healthcare sector. Over the past several years, a lot of effort has been put to educate people, treat and prevent infection with Tuberculosis with the help of National Health Schemes.

Yet, India has the highest number of Tuberculosis and Multiple-Drug-Resistant Tuberculosis cases. Out of all individuals affected with Tuberculosis worldwide, India itself bears the burden of nearly 24% of cases.

As per Government statistics, Tuberculosis is one of the leading causes of death in India. Each year, a surprising number of nearly 280,000 new cases of Tuberculosis are reported in India.

What is a Tb Skin Test?

A Tb skin test is also called the Mantoux test or a Tuberculin test. It is performed not only to diagnose Tb but also for the screening of people who frequently come in contact with Tb patients such as health care workers, residents of a location where Tb is rampant or foreign-born people from countries who have a high incidence of Tb. The procedure for performing a Tuberculin or Mantoux test is

1. The Tuberculin test kit includes a vial, a disposable syringe for administering the dose and a ruler to measure induration is additionally used.
2. A standard dose of 5 units of Tuberculin is injected intradermally (into the skin) over the arm right below the elbow in a palm-facing-above position.
3. The result of this test is observed between 48-72 hours.
4. The reaction is estimated by measuring the diameter of induration in millimeters. No induration is written as “0 mm”.
5. Redness or eythema is not considered as a parameter to diagnose Tb using a Tuberculin test.
6. In elderly individuals, however, results may take more than 72 hours to appear.

Analyzing the difference between a positive and negative Tb skin test is quite difficult for a lay-person. A table later in the article will help you understand the difference between a positive and negative Tb skin test.

What is a Positive Tb Skin Test?

An induration (swelling) of more than 5 mm is considered to be positive under certain circumstances.

What is a Negative Tb Skin Test?

A negative Tb skin test means that either a person is not infected with the bacillus or the infection is too recent and not too much time has elapsed for the body to react to a Mantoux test. Induration of skin less than 5 mm is considered negative for Tuberculosis.

Difference Between Positive and Negative Tb Skin Test

A positive skin test can be seen and interpreted according to diameter of induration.

Once a person has been tested positive for Tuberculosis, he/she may have to undergo further investigations to have a confirmatory diagnosis of Tuberculosis. A negative skin test result on the other hand can be divided into false-positive and false-negative for the purpose of understanding

False Positive Test Results

A false positive test result means a Tb skin test appears positive even though the person is not infected with Mycobacterium Tuberculosis. False positive results for Tb skin test can be seen in

• Infection with a non-tuberculosis causing Mycobacterium.
• History of receiving BCG vaccine.
• Fault in the administration of Tuberculin.
• Faulty interpretation of reaction.
• Touching the skin over injected area causing itching and swelling is also a possible cause for a false-positive result
• Hypersensitivity or an allergic reaction to Tuberculin

In the above mentioned conditions, the reading should be considered as negative unless proved otherwise from some other tests to diagnose Tb.

BCG and Tb Skin Test:

• Persons who have received BCG vaccine previously show a false-positive result for several years.
• According to US department of health, BCG vaccine does not give immunity against military tuberculosis and tuberculous meningitis.
• Therefore a person who has received BCG vaccine and tests positive for Tb skin test is considered to have latent Tuberculosis.
• If BCG has been given during infancy, it will not affect test results of Mantoux test done during adulthood.

False Negative Test Result

A negative test result means that a person has never been exposed to Mycobacterium Tuberculosis. A negative result may also be seen if the ability to react to Tuberculin is diminished. Immuno-compromised persons frequently have a low ability to react to Tuberculin. False negative results can be seen in

• Cutaneous anergy (inability to react to skin test due to weak immune system)
• Recent Tb infection – less than 8-10 weeks
• Old Tb infection
• Infectious mononucleosis
• Less than 6 months of age.
• Recent immunization against measles, mumps, rubella and polio
• Malnutrition
• Upper respiratory tract infection.
• Immuno compromised patients
• Patients having Hodgkin’s disease
• Persons receiving corticosteroid therapy
• Other viral illnesses like measles and chicken pox.
• Incorrect administration of Tuberculin.

Human errors in the administration of Tuberculin and interpretation of results often make it difficult to differentiate between a positive and negative Tb skin test. In such cases of a false-negative or a false-positive reaction skin test may have to be repeated again along with other investigations either support or rule out the diagnosis or Tuberculosis.

The reaction of the body to a dose of tuberculin injected intradermally is a type of delayed hypersensitivity response. However, no correlation has been found between the size of induration and likelihood of an active TB illness.

The test has a low predictive value for detecting active Tb illness. And therefore it has been taken over by more specific investigations like Chest X-ray and sputum examination for the purpose of accurate diagnosis.